This invention relates to the fabrication of semiconductor devices, and, more particularly, to the deposition of an epitaxial layer on the surface of an indium phosphide substrate.
The loss of phosphorus from the InP substrate surface, due to surface dissociation during the homogenization portion of the growth cycle using liquid phase epitaxy (LPE), results in the formation of indium droplets on the substrate surface. These droplets cause localized variations in the melt composition, especially when growing alloys such as InGaAsP, and further produce localized regions with solidus composition variations. The formation of these indium droplets is clearly counterproductive to the goal of having a substantially planar surface into which other semiconductor materials can be grown. As a result the surface of the InP is no longer smooth and planar, and any epitaxial layer grown thereon will both conform to the surface roughness and contain crytallographic defects which propagate from the damaged substrate surface. Almost all semiconductors require substrates and epitaxial layers thereon that are planar and defect-free as possible. Thus, these surfaces must be free of undesired surface roughness and defects so that quality semiconductor devices can be made therefrom.
A variety of methods have been attempted to eliminate indium droplets and the defects arising therefrom, such as placing an InP cover wafer over the InP substrate surface to prevent the phosphorus from vaporizing, using phosphine gas as shown in U.S. Pat. No. 4,263,064, entitled, "Method of Liquid Phase Epitaxial Growth", or using an indium wipe melt to clean the substrate. These methods work but either affect the growth process with other unwanted problems or do not substantially eliminate the indium droplets.